There are two major goals in this proposed five year continuation which is currently evaluating genetic and environmental influences on behavior during middle childhood, employing more than 600 adopted and non-adopted children taking part in the Colorado Adoption Project (CAP), a longitudinal, prospective adoption study founded in 1977 and currently funded for testing through 16 years of age. The first goal is to complete testing of the 7-year-old CAP children, including 198 adopted and non-adopted siblings. Seventy children will be tested at age 7 in this part of the project. The second goal is to complete testing of 303 monozygotic and dizygotic twin pairs who have previously taken part in the Twin Infant Project (TIP), a study of cognitive developing during infancy involving twins and their parents, on whom there are considerable follow-up data relevant to the CAP test battery at 1, 2 and 3 years of age. This part of the project will involve testing 120 and 303 twin pairs at ages 4 and 7, respectively. The CAP adult, three hour test battery includes measures of the major factors of cognitive abilities and personality, as well as interviews and questionnaires pertaining to family background, common medical and behavioral problems, interests and talents, and frequently used drugs. For the children, the assessments employ standard tests of cognitive and language development, personality/temperament, motor development and health. Environmental assessments in the adoptive and control homes are emphasized. The adopted and control probands and their younger siblings are studied in their homes at 1, 2, 3 and 4; in the laboratory at 7 and 12; and via telephone interviews at 9, 10 and 11. Testing at 13 through 16 years is currently taking place in the laboratory and the home. State-of-the-art model-fitting procedures have been developed to exploit fully the multivariate complexity of the data resulting from this test battery. Thus, the overall goal of the proposed research is to create a firmly established study that incorporates a combined adoption, sibling and twin design into a prospective, longitudinal study of behavioral development. A study of this magnitude and quality has never before been undertaken, and it is most unlikely that the opportunity to undertake such an ambitious study will ever occur again. The proposed continuation will substantially advance basic research in child development and will have important implications for child-rearing practices, education and mental health.